1. Field of the Invention
The present invention relates to image forming apparatus such as copying machines, laser beam printers or liquid crystal printers, and more particularly to image forming apparatus including a process cartridge which has incorporated therein at least an electrophotographic photosensitive member or like image bearing member and which is removably and replaceably installed in the body of the apparatus.
2. Description of the Related Art
With electrophotographic image forming apparatus, the electrostatic and physical characteristics of the material of the photoconductive layer formed on the photosensitive drum are a factor for determining the design specifications (as to image forming conditions) such as the image forming velocity of the apparatus. Generally, therefore, the type of photosensitive member to be used differs with the image forming velocity. For example, photosensitive drums coated with an inorganic photoconductive material, such as .alpha.-Si or Se, are used for image forming apparatus set to a high image forming velocity (hereinafter referred to as "high speed machines"), while photosensitive drums coated with an organic photoconductive material are employed for apparatus set to a relatively low image forming velocity (hereinafter referred to as "low speed machines"). However, photosensitive drums of either type deteriorate in electrostatic or physical characteristics with repetitions of image forming operation, so that the drum which has served for life, as estimated from the number of repetitions of image formation, is replaced by a new one. Further the expendables which must be replaced for use in image forming apparatus include the developing unit, cleaner, etc. in addition to the photosensitive drum.
Image forming apparatus are already known which have a cartridge removably installed therein and incorporating a charger, photosensitive drum, developing unit and cleaner as arranged together within a frame so as to facilitate the replacement of such expendables (U.S. Pat. No. 4,607,941). With the known apparatus, the cartridge in service is replaced by new one, for example, with reference to the life of the drum as a standard. The charger for use in charging the surface of the photosensitive drum is a scorotron charger which comprises a wire electrode for effecting corona discharge, a shield member partly surrounding the electrode and a grid electrode grounded via a constant-voltage device (varistor). With this charger, corona discharge produces a corona current, and the corona current flowing through the grid electrode is passed through the constant-voltage device to maintain the grid electrode at a predetermined potential. Thus, the charge potential of the photosensitive drum is dependent on the potential of the grid electrode. The charge potential of the drum is dependent also on the image forming velocity, i.e., on the peripheral speed of the drum. When the grid potential is at a given level, the higher the peripheral speed, the lower is the drum charge potential.
With image forming apparatus having a cartridge as described above, there arises a need to reduce the production cost in recent years by using common expendables for apparatus which are different in design specifications, in addition to facilitating the replacement of expendables. For example, in the case where an existing product (e.g., low speed machine) is to be modified for sophistication to commercially provide a new product (e.g., high speed machine), it is desired to make expendables for the new product usable also for the existing product. In this case, the photoconductive material for coating the photosensitive drum or the constant-voltage device (varistor) to be connected to the grid electrode must be a material or a device of rated value which is adapted for use in the high speed machine so that the drum surface can be charged to the desired potential even when the drum is rotated at a high speed.
With conventional image forming apparatus, nevertheless, the process cartridge for use in a particular apparatus is in conformity with the design specifications (e.g., image forming velocity) of that apparatus only. Accordingly, if the process cartridge for the high speed machine is used for the low speed machine, the surface of the photosensitive drum is charged excessively and results in various objections. For example, when a two-component developer containing a carrier and a toner is used for reversal development, the carrier is deposited on edge portions of images of small area such as characters or on nonimage areas to smudge the copy images. Such deposition of carrier varies the standard toner-to-carrier ratio of the developer, leading to the supply of an excess of toner and consequently failing to triboelectrically charge the toner with the carrier effectively. The toner which is not charged as desired fails to give copy images of desired image density. The toner further becomes prone to spill over and soil the detection faces of sensors which are arranged at various locations in the interior of the image forming apparatus to result in detection errors. To obviate these objections, there is a need to replace the constant-voltage device provided in the low speed machine by one having a rated value for use in the high speed machine, whereas the replacement requires dispatch of a special serviceman and imposes an excessive burden on the user.